For most ships of such design the condition of sailing in relative large stern quartering or following waves can become dangerous due to the occurrence of “broaching”. Broaching is the peculiar ship behaviour, which consists of a coupled yaw-, sway-and roll motion. Ships may experience this broach motion in the described wave condition and once it occurs it can lead to seriously large roll angles and eventually end in a “capsize”.
The broaching phenomenon may be roughly explained as follows: In stern quartering and following waves the ship stern is lifted asymmetrically by the incoming wave and it starts the ship to heel (and also pitch). Broaching is particularly prone in waves with a wave length close to the ship length. So simultaneously the ship puts its bow in the face of the (next) wave. Caused by the asymmetry of the hull due to the introduced heeling angle and the directionally destabilizing effect of the now deeply immersed bow sections the ship starts to yaw. Combined with the forward speed this may lead to an increase in the heeling angle which on its turn worsens the hull asymmetry and therewith to further course instability. This may lead to the ship coming beam side to the waves in principal a potentially dangerous situation as it may lead to an even further increase in heel.
With fast ships this broaching phenomenon may be en-countered more frequently than with regular ships and may have more serious effects because fast ships are generally smaller and sail therefore in relatively larger waves. Also the high forward speed worsens the heeling influence induced by the centrifugal forces once the ship is in a turn and the encounter frequency between the ship and the following waves. In particular the longer and higher waves may have a lower frequency making their impact bigger.
Ships are normally directionally controlled by either a helmsman or an autopilot. In general the applied control is aimed at keeping the ship on a preset course. From experience, full scale and model scale measurements it is known that this directional control applied in following waves worsens the broaching behavior of the ship, due to the unfavourable phase between the steering force applied and the herewith induced heeling moment.